Mason-Pfizer Monkey virus (M-PMV) is an exogenous, primate retrovirus that was isolated from a breast carcinoma of a rhesus monkey. It is the prototype virus of the D-type retroviruses which, like the B type retrovirus mouse mammary tumor virus, preassemble a complete capsid within the cytoplasm of a infected cell. These A-type particles migrate to the plasma membrane and are enveloped by a lipid bilayer upon release from the cell. The major goals of this proposal are to define those viral coded gag-gene functions that are essential in assembly, morphogenesis and infectivity of the M-PMV particle. The carboxy-terminal amino acid sequence of the five major gag gene products of this virus will be determined. This information will be used together with amino-terminal amino acid sequence information to define the polypeptide coding regions on the nucleic acid sequence of the gag gene determined from cloned M-PMV DNA. In vitro site directed mutagenesis of the gag gene coding region and of a sequence 3' of gag which may code for the viral protease will be used to determine the function of these regions. Deletion, insertion, and point mutagenesis procedures will be employed for these studies. The role of the envelope glycoproteins in directing the budding and release of viral particales will be investigated utilizing similar procedures.